Low levels of medication adherence represent a growing problem for global health systems. We report evidence from a pre-registered randomized controlled trial aimed at increasing adherence rates and delivered through 278 UK pharmacies. Patients (N=16,191) were asked to commit to taking their medication as prescribed by signing their name on a sticker designed to attach to their medication packaging. In two additional trial arms, the commitment was paired with a message describing the negative consequences arising from non-adherence; either the increased risk to the patient’s own health, or the financial costs to society. Our results indicate that for participants who signed the pre-commitment without reference to the negative consequences arising from non-adherence, there was no change to their medication adherence levels in comparison to the control group. However, participants who signed a pre-commitment paired with the health warning were significantly more likely to adhere to their medication than the control group (odds ratio = 1.59, CI95% [1.02; 2.48]). Our results provide new insights into the reasons underlying medication adherence and the effective design of pre-commitment contracts.